In recent years, organic electroluminescent elements (hereinafter also referred to as “organic EL elements”) have been under extensive development and have been used in display devices and lighting devices.
Organic EL elements are thin, completely solid elements that can emit light at a low voltage of about several volts to several tens of volts, and have many advantages, such as high luminance, high emission efficiency, small thickness, and light weight. Such an organic EL element includes a pair of electrodes and a light emitting unit composed of an organic material and disposed between the electrodes. Light emitted from the light emitting unit is extracted to the outside through the electrodes.
In a typical technique for obtaining light having a desired color from an organic EL device, organic EL elements which emit light having different colors (e.g., red, green, and blue) are disposed in a stripe pattern, and the intensity of light emitted from each of the organic EL elements is controlled to produce light of any color. The technique involves a separate coating process in which organic EL elements which emit light having different colors are individually patterned on a substrate with a highly precise metal mask, or a color filter process in which color filters are disposed on a substrate in any pattern, and a white light-emitting organic EL element is disposed on the color filters.
Unfortunately, these two processes require provision of individual emission regions on a light-emitting surface, and also require provision of non-emission regions between adjacent emission regions for avoiding mixing of colors of light. Thus, the processes cause a problem in that the ratio of the emission regions to the entire light-emitting surface (aperture ratio) is very low, resulting in a significant reduction in emission efficiency. In addition, the processes require complicated production steps, leading to an increase in production cost.
In order to solve these problems, a process has been proposed which involves lamination of light emitting units which emit light having different colors in a thickness direction of an organic EL element via intermediate electrodes, rather than the planar arrangement of organic EL elements which emit light of different colors. This process allows the light emitting units to emit light independently for emission of light having any color (see, for example, PTL 1).
According to this process, the aperture ratio of each light emitting unit can be approximated to substantially 100%, resulting in a significant increase in emission efficiency. Furthermore, this process does not require patterning with a highly precise metal mask or formation of a color filter, and thus enables production steps to be simplified, probably leading to a reduction in production cost or an increase in yield.
This lamination process is more advantageous than the aforementioned two processes in terms of emission efficiency and production process, particularly in the case where the color of emitted light is appropriately varied in a device such as a lighting device which has a relatively large emission region and which does not require pixels.
Unfortunately, this lamination process poses the following problems:
An intermediate electrode disposed between each pair of adjacent light emitting units desirably has a small thickness for effective extraction of light emitted from each of the light emitting units. Specifically, the thickness of the intermediate electrode, which may vary depending on the material thereof, is desirably 20 nm or less.
The organic EL element prepared by this process includes several layers (including the light emitting unit) laminated between the intermediate electrode and the substrate. Thus, the intermediate electrode needs to be patterned to extend over a step between the layers for connection of the intermediate electrode to an external power source. If the intermediate electrode has a thickness of 20 nm or less as described above, the intermediate electrode may break at the step between the layers, or the intermediate electrode may be thinned at the step, resulting in uneven thickness of the intermediate electrode or an increase in resistance.
Thus, the lamination process has a difficult problem of achieving both high translucency and high conductivity of the intermediate electrode.